(a) Field of the Invention
The present invention relates to a lens system for compact cameras having a short total length, and more specifically to a photographic lens system having a field angle of at least 60.degree., an F number of 2.8, a telephoto ratio smaller than 0.1 and a very short total length.
(b) Description of the Prior Art
Lens systems for use with compact cameras which use 35 mm size of films generally have field angles around 63.degree.. In order to design lens systems of this class so as to have telephoto ratios smaller than 0.1 and very short total length, it is necessary to locate the back principal point of the lens systems on the object side of the first lens surface. In order to realize such a location of the back principal point, it is considered to adopt the type generally used as telephoto lens systems in which the front lens group is designed as a deverging lens group and the rear lens group is designed as a converging lens group. As a conventional example of this type of lens system, there has already been known the one disclosed by Japanese Unexamined Published Patent Application No. 76147/79. This lens system has a filed angle of 63.degree., an F number of 2.8, a telephoto ratio smaller than 1.0 and favorably corrected aberrations. However, this lens system is not so compact since it comprises a positive lens component having a large diameter. Lens systems similar to this lens system are disclosed by Japanese Unexamined Published Patent Application No. 22215/82 and No. 30814/82. These conventional examples also have a tendency similar to that of the one disclosed by Japanese Unexamine Published Patent Application No. 76147/79.
As an improved type of the above-mentioned comventional example, Japanese Unexamined Published Patent Application No. 73014/80 discloses a lens system comprising a concave lens component arranged on the extreme image side. This lens system comprises four lens components, i.e., a first positive meniscus lens component having a convex surface on the object side, a second biconcave lens component, a third biconvex lens component and a fourth negative meniscus lens component having a convex surface on the image side. Said lens system has a field angle of 63.degree., a telephoto ratio smaller than 1.0 and favorably corrected aberrations, and diameter of said lens component arranged on the extreme image side is not so large. However, the lens system has an F number of 3.5 and a relative aperture which is smaller than those of the other conventional examples. If it is attempted to enlarge the relative aperture of this lens system to an F number of 2.8, high order spherical aberration will be remarkably aggravated, and if it is attempted to correct the aberrations with the telephoto ratio and field angle maintained at 0.1 and 60.degree. or wider respectively, it will be impossible to maintain good balance among the aberrations.